upgrade checkstyle

updates checkstyle and our corresponding code check configuration to the newest version

todo:

• wait for eclipse plugin support for 8.41.1: https://github.com/checkstyle/eclipse-cs/issues/282
• suggestion by @rafael (in chat): configure checkstlye as execution goal in verify phase, so the eclipse plugin can auto-detect settings
• clarify if it's OK to skip mvn clean in the Reports stage.